All major pipelines are protected by cathodic protection, which has been very successful in markedly decreasing failures due to corrosion. During pipe coating applications, some breaks or imperfections in the coating will occur, so the imposed negative charge on the pipelines protects these areas where "holidays" exist in the coating or where holidays develop later. Unfortunately, with the aid of moisture and mineral matter in the soil, the negative charge increases undercutting and disbonding of the coating system at the holidays at a faster rate than would occur if the charge were not present. Loose coating increases the electical power requirements on a pipeline and in badly damaged areas even corrosion may result due to the difficulty of maintaining the proper charge on all areas of the pipe.
A primer for asphaltic pipe coatings can serve at least three important functions:
1. It can provide better adhesion between the hot applied pipe coating and the cooler pipe during application.
2. It can prevent tiny voids under the pipe coating by filling in pits and irregularities in the rough steel surfaces that the molten coating may not fill.
3. It can provide a different degree of physical and chemical protection than is available by the pipe coating alone. Such factors as cathodic disbonding resistance and impact resistance, for example, are often greatly influenced by the primer used on the pipe.
However, the presently available primers, including the chlorinated rubber base primers, have not proved to be completely satisfactory.